Ion implanters are used in the manufacture of semiconductor devices and other materials. In such ion implanters, semiconductor wafers or other substrates are modified by implanting atoms of a desired species into the body of the wafer, for example to form regions of varying conductivity.
Ion implanters are well known and generally conform to a common design as follows. An ion source generally comprises an arc chamber in which a hot plasma is generated. The plasma will contain ions of a desired species to be implanted.
An extraction lens assembly produces an electric field that extracts ions from the ion source and forms a mixed beam of ions. Only ions of a particular species are usually required for implantation in a wafer or other substrate, for example a particular dopant for implantation in a semiconductor wafer. The required ions are selected from the mixed ion beam that emerges from the ion source by using a mass analysing magnet in association with a mass resulting slit. By setting appropriate operational parameters on the mass analysing magnet and the ion optics associated therewith, an ion beam containing almost exclusively the required ion species emerges from the mass resolving slit. The ions travel along a flight tube as they pass through the mass analysing magnet.
The ion beam is transported along a beam line to a process chamber where the ion beam is incident on a substrate held in place in the ion beam path by a substrate holder. When a substrate is held clear of the ion beam path, the ion beam strikes a beamstop.
The various parts of the ion implanter are operated under the management of a controller, typically a suitably trained person, a programmed computer, or the like. A more detailed description of an ion implanter of this general type can be found in U.S. Pat. No. 4,754,200.
During normal operation of an ion implanter, unwanted material may become entrained in the ion beam. This material may strike the substrate causing contamination or even damage to the devices being formed on the substrate. A major source of contaminants is material from ion implanter components that surround the ion beam path. If the ion beam strikes such components, material may be sputtered from that surface. As the surfaces surrounding the ion beam path are typically made from graphite, graphite is a major component in the material entrained in the ion beam.
Entrained material may be conveyed directly to the substrate to be implanted, or it may adhere to another surface in the ion implanter. Surfaces adjacent to the ion beam are most prone to receiving such deposits. As the amount of material deposited accumulates, the chances of the deposits delaminating to form flakes or particles increases. These flakes or particles frequently detach from their host surface and may become entrained in the ion beam. Consequently, the flakes or particles contain sputtered material that still ultimately reaches the substrate.
Our co-pending U.S. patent application Ser. No. 11/651,107 and US Patent Application Publication No. 2007/0102652 address the problem of material depositing on downstream surfaces to form large flakes. The present invention is concerned with the initial sputtering of material and how to reduce the amount of material entrained in the ion beam. In particular, the present invention is concerned with sputtering of material from the flight tube.
A further problem is the liberation of material from the beamstop when it is struck by the ion beam. The close proximity of the substrate to the beamstop exacerbates the problem of contamination from the beamstop.
The present invention also finds application in so-called beam parks or flag Faradays, devices located part way along a beam line in an ion implanter to act as a beam dump.